Seismic data acquisition with varying distance between seismic vessels

ABSTRACT

A method for acquiring seismic data comprises operating a seismic vessel towing at least one of a seismic streamer with a plurality of seismic sensors and a seismic source. The inline distance between the vessel and one or more other vessels is adjusted according to a predefined function, where the seismic vessels travel along offset seismic acquisition lines, by increasing and reducing the in-line distance during the acquisition of the seismic data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/564,363, filed Dec. 9, 2014, issuing Aug. 1, 2017 as U.S. Pat. No.9,720,120, SEISMIC DATA ACQUISITION WITH VARYING RELATIVE DISTANCEBETWEEN MULTIPLE SEISMIC VESSELS, and claims priority to U.S.Provisional Application No. 61/914,836, SEISMIC DATA ACQUISITION WITHVARYING RELATIVE DISTANCE BETWEEN MULTIPLE SEISMIC VESSELS, filed Dec.11, 2013, each of which is hereby incorporated by reference herein, inthe entirety and for all purposes.

BACKGROUND

The present invention generally relates to marine seismic prospecting,and in particular to seismic prospecting methods using multiple vessels.

Petrochemical products such as oil and gas are ubiquitous in society andcan be found in everything from gasoline to children's toys. Because ofthis, the demand for oil and gas remains high. In order to meet thishigh demand, it is important to locate oil and gas reserves in theEarth. Scientists and engineers conduct “surveys” utilizing, among otherthings, seismic and other wave exploration techniques to find oil andgas reservoirs within the Earth. These seismic exploration techniquesoften include controlling the emission of seismic energy into the Earthwith a seismic source of energy (e.g., dynamite, air guns, vibrators,etc.), and monitoring the Earth's response to the seismic source withone or more receivers in order to create an image of the subsurface ofthe Earth.

Each receiver may include, for example, a pressure sensor and/or aparticle motion sensor in proximity to one another. The pressure sensormay be, for example, a hydrophone that records scalar pressuremeasurements of a seismic wavefield. The particle motion sensor may be,for example, a three-component geophone that records vectorial velocitymeasurements of the seismic wavefield. By observing the reflectedseismic wavefield detected by the receiver(s) during the survey, thegeophysical data pertaining to reflected signals may be acquired andthese signals may be used to form an image indicating the composition ofthe Earth near the survey location.

Marine seismic surveys generally involve towing one or more streamercables comprising a plurality of receivers with a seismic vessel. Thenumber of receivers placed in the streamer and the relative distancebetween the receivers generally determines the quality of seismic datathat is recorded. Improving seismic data collection has traditionallyinvolved increasing the length of streamer cables and the density ofreceivers included therein. However, the longer the streamer cables, themore difficult it becomes to keep streamer cables separated and in adesired configuration, for example, when turning the seismic vessel ormaneuvering the seismic vessel in icy or obstructed waters.

SUMMARY

The present invention generally relates to marine seismic prospecting,and in particular to seismic prospecting using multiple vessels. Duringacquisition of seismic data, a distance between a first seismic vesseland a second seismic vessel may be adjusted according to a predefinedfunction such that data is collected at a variety of offsets and/orazimuths.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features, advantages andobjects of the present invention are attained and can be understood indetail, a more particular description of the invention, brieflysummarized above, may be had by reference to the embodiments thereofwhich are illustrated in the appended drawings.

FIGS. 1A-C illustrate an exemplary seismic survey according to anembodiment of the invention.

FIGS. 2A-C illustrate exemplary distance functions according toembodiments of the invention.

FIG. 3 illustrates dynamically adjusting distance functions according toan embodiment of the invention.

FIGS. 4A-C illustrate exemplary relative positioning between seismicvessels according to embodiments of the invention.

FIG. 5A illustrates adjusted distance functions during multiple passesover a navigation line, according to an embodiment of the invention.

FIG. 5B illustrates exemplary seismic receiver coverage over an area ofinterest during multiple passes over a navigation line, according to anembodiment of the invention.

FIG. 6 illustrates an exemplary control system according to anembodiment of the invention.

FIG. 7 is a flow diagram of exemplary operations performed during aseismic survey, according to an embodiment of the invention.

DETAILED DESCRIPTION

In the following, reference is made to embodiments of the invention.However, it should be understood that the invention is not limited tospecific described embodiments. Instead, any combination of thefollowing features and elements, whether related to differentembodiments or not, is contemplated to implement and practice theinvention. Furthermore, in various embodiments the invention providesnumerous advantages over the prior art. However, although embodiments ofthe invention may achieve advantages over other possible solutionsand/or over the prior art, whether or not a particular advantage isachieved by a given embodiment is not limiting of the invention. Thus,the following aspects, features, embodiments and advantages are merelyillustrative and are not considered elements or limitations of theappended claims except where explicitly recited in a claim(s). Likewise,reference to “the invention” shall not be construed as a generalizationof any inventive subject matter disclosed herein and shall not beconsidered to be an element or limitation of the appended claims exceptwhere explicitly recited in a claim(s).

One embodiment of the invention is implemented as a program product foruse with a computerized system. The program(s) of the program productdefines functions of the embodiments (including the methods describedherein) and can be contained on a variety of computer-readable media.Illustrative computer-readable media include, but are not limited to:(i) information permanently stored on non-writable storage media (e.g.,read-only memory devices within a computer such as CD-ROM disks readableby a CD-ROM drive); (ii) alterable information stored on writablestorage media (e.g., floppy disks within a diskette drive or hard-diskdrive); and (iii) information conveyed to a computer by a communicationsmedium, such as through a wireless network. The latter embodimentspecifically includes information downloaded from the Internet and othernetworks. Such computer-readable media, when carrying computer-readableinstructions that direct the functions of the present invention,represent embodiments of the present invention.

In general, the routines executed to implement the embodiments of theinvention, may be part of an operating system or a specific application,component, program, module, object, or sequence of instructions. Thecomputer program of the present invention typically is comprised of amultitude of instructions that will be translated by the native computerinto a machine-readable format and hence executable instructions. Also,programs are comprised of variables and data structures that eitherreside locally to the program or are found in memory or on storagedevices. In addition, various programs described hereinafter may beidentified based upon the application for which they are implemented ina specific embodiment of the invention. However, it should beappreciated that any particular program nomenclature that follows isused merely for convenience, and thus the invention should not belimited to use solely in any specific application identified and/orimplied by such nomenclature.

FIG. 1A illustrates an exemplary seismic survey according to anembodiment of the invention. As illustrated in FIG. 1A, a first seismicvessel 110 and a second seismic vessel 120 may collaboratively perform aseismic survey. In one embodiment, the first seismic vessel 110 may beconfigured to tow one or more seismic sources 111 and one or morestreamer cables 112. The second seismic vessel 120 may also beconfigured to tow one or more seismic sources 121 and/or one or morestreamer cables 122, as shown in the embodiment illustrated in FIG. 1A.

While FIG. 1A shows the first vessel 110 and the second vessel 120 eachtowing respective sources and streamer cables, in alternativeembodiments, the sources and streamers may be arranged differentlybetween the first and second vessels. For example, in one embodiment,one of the first vessel 110 and the second seismic vessel 120 may towone or more sources and one or more streamer cables, while the other oneof the first seismic vessel 110 and second vessel 120 may only tow oneor more streamer cables. In another embodiment, one of the first seismicvessel 110 and second seismic vessel 120 may tow one or more sources andone or more streamer cables, while the other one of the first seismicvessel 110 and second seismic vessel 120 may only tow one or moresources. In yet another embodiment, one of the first seismic vessel 110and second seismic vessel 120 may only tow one or more sources, whilethe other one of the first seismic vessel 110 and second seismic vessel120 may only tow one or more streamer cables.

In one embodiment, the first seismic vessel 110 and the second seismicvessel 120 may be configured to only tow one or more sources each. Insuch an embodiment, an additional seismic vessel may also be included inthe survey to tow one or more streamer vessels near the first and secondseismic vessel. In another embodiment, the first seismic vessel 110 andthe second seismic vessel may be configured to only tow one or morestreamers. In such an embodiment an additional seismic vessel may beincluded in the survey to tow one or more sources near the first seismicvessel and second seismic vessel.

Referring back to FIG. 1A, the seismic sources 111 and 121 may each bean air gun array configured to release a blast of compressed air intothe water column towards the seabed 130. A blast of compressed air fromthe air guns 111 and/or 121 generates seismic waves which may traveldown towards the seabed 130, and penetrate and/or reflect fromsub-seabed surfaces. The reflections from the sub-surfaces may berecorded by seismic sensors 113 as seismic data. Exemplary seismicsensors include any one or a combination of hydrophones, geophones,particle motion sensors such as accelerometers, and the like. Theseismic data acquired via the seismic sensors 113 may be processed todevelop an image of the sub-surface layers. These images may be analyzedby geologists to identify areas likely to include hydrocarbons or othersubstances of interest.

In one embodiment of the invention the seismic sources 111 and 121 maybe configured to operate simultaneously or substantially simultaneously.Simultaneous source operation generally involves firing two sources inan overlapping manner with a predefined time delay. Interference betweensources can also be created by firing sources simultaneously atpredefined spaced apart locations between the sources. Multiple sourcesthat are fired simultaneously (or nearly simultaneously) from differentlocations may provide better coverage, e.g., in obstructed areas, andmay provide greater azimuthal diversity for the survey.

In one embodiment of the invention, the size of the seismic receiverarrays (formed by the cables 112 and 122) towed by the seismic vesselsmay be the same. However, in alternative embodiments, each vessel maytow a different sized array. Examples of factors determining array sizeinclude one or more of the number of cables, relative spacing betweencables, the length of the cables, and the like.

FIG. 1A further illustrates a distance D that is maintained between thefirst vessel 110 and the second vessel 120 during seismic dataacquisition. In one embodiment of the invention, the distance D may bedetermined by a predefined variable function. The predefined functionmay be configured to adjust a parameter, for example, speed, velocity,acceleration, or the like of two or more seismic vessels such that thedistance between the seismic vessels is varied in a desirable manner.The predefined function is sometimes referred to herein as a “distancefunction” or “predefined distance function”.

For example, in one embodiment, the distance between the first vessel110 and the second vessel 120 may be reduced from a first distance to asecond distance during a first time period. Thereafter, the distancebetween the first vessel 110 and the second vessel 120 may be increasedfrom the second distance to the first distance in a second period oftime. The distance between the first vessel and the second vessel may becontinuously varied by repeatedly moving the first vessel 110 and thesecond vessel 120 closer and then further away, in one embodiment.

The distance D may be determined based on any two predefined points on,for example, the seismic vessels 110 and 120, the seismic cables orarrays 112 and 122, sources 111 and 121, or the like. In a particularembodiment, the distances may be determined based on any reasonablepredefined points on the first vessel 110 and second vessel 120, e.g.,the bow, stern, center of the vessel, etc. In one embodiment, GPSdevices may be provided at one or more locations associated with theseismic vessels or items being towed by the seismic vessels. Thedistance between may therefore be determined based on the relativedistance between the GPS devices.

FIG. 1B illustrates a seismic survey according to another embodiment ofthe invention. As shown in FIG. 1B, a first seismic vessel 150 and asecond seismic vessel 170 may tow respective seismic sensor arrays 151and 171. A third seismic source vessel 180 may tow a seismic sourcearray 181. During acquisition, the vessels 150 and 170 may vary adistance between them based on a predefined distance function D. In oneembodiment, the source vessel 180 may travel at a substantially constantvelocity associated with the average velocities of the vessels 150 and170 and provide the source impulses for recording seismic data in thesensor arrays 151 and 171. In an alternative embodiment, the sourcevessel 180 may vary its respective velocity while maintaining an averagevelocity associated with the average velocities of the vessels 150 and170.

FIG. 1C illustrates a seismic survey according to another embodiment ofthe invention. As shown in FIG. 1C, a first source vessel 191 and asecond source vessel 192 may tow respective seismic source arrays 193and 194. A third seismic streamer vessel 195 may tow a seismic sensorarray 196. During acquisition, the vessels 191 and 192 may vary adistance between them based on a predefined distance function D. In oneembodiment, the seismic streamer vessel 195 may travel at asubstantially constant velocity associated with the average velocitiesof the vessels 191 and 192. In an alternative embodiment, the seismicstreamer vessel 195 may vary its respective velocity while maintainingan average velocity associated with the average velocities of thevessels 191 and 192.

FIGS. 2A and 2B illustrate exemplary functions to vary the distancebetween seismic vessels, according to an embodiment of the invention.FIG. 2A illustrates sinusoidal variation of the distance D between thefirst vessel 110 and the second vessel 120 from a distance d1 to adistance d2. The distances d1 and d2 may be any preselected distances.For example, in one embodiment, the distance d1 may be at or near 0(zero) or as close as operationally possible for one seismic vessel tobe near the other. FIG. 2B illustrates an alternative distance function,wherein the distance D between the first vessel 110 and second vessel120 is varied in a linear fashion. Any reasonable slope of inclinationand declination may be used, and moreover, the slope of inclination neednot have the same magnitude as the slope of declination.

While FIGS. 2A and 2B illustrate continuous and periodic distancefunctions, embodiments of the invention are not limited to suchfunctions. In general, the distance function may be any type offunction, whether periodic or aperiodic. FIG. 2C illustrates anexemplary distance function that is not periodic. The function may beused for establishing distance between a first seismic vessel 110 and asecond seismic vessel 120, for example, while acquiring seismic data ona predetermined navigation line.

In one embodiment of the invention, different distance functions may bedefined for different types of areas of interest while conducting theseismic survey. For example, FIG. 3 illustrates the seismic vessels 110and 120 approaching an area of interest 310. FIG. 3 also illustratescorresponding distance functions that may be implemented by the vessels110 and 120 while conducting the survey. For example, in a first timeperiod T1 when the seismic vessels are outside a zone Z associated withthe area of interest 310, the seismic vessels may operate based on afirst distance function f1. Upon entering the zone Z, the seismicvessels 110 and 120 may begin operating according to a second distancefunction f2. Thereafter, upon exiting the zone Z, the seismic vessels110 and 120 may return to operating according to the distance functionf1.

Any number of different functions may be defined for any number ofdifferent types of areas of interest. In general the different distancefunctions may be defined so that a desired density of sensors isachieved for shot gathers in different types of areas of interest. Forexample, in FIG. 3, the area of interest 310 may have a high probabilityof containing hydrocarbons, and therefore a greater density may bedesired for the shot gather to develop a more detailed and more reliableimage of the sub surface. Accordingly, as shown in FIG. 3, the seismicvessels 110 and 120 may be operated relatively closer to one another toimprove density of the shot gathers.

FIGS. 4A and 4B illustrate exemplary relative positioning of the seismicvessels 110 and 120. In one embodiment, the seismic vessels 110 and 120may be configured to move along the same navigation line L1, asillustrated in FIG. 4A. Therefore, the distance function D may cause thefirst vessel 110 and second vessel 120 to move closer together and/orfurther away along that same line L1. In an alternative embodiment, thefirst seismic vessel 110 may be configured to travel on a first line L2,and the second seismic vessel 120 may be configured to travel on asecond line L3 that is offset from the first line L2, as illustrated inFIG. 4B. The offset O may be any reasonable distance. In one embodiment,the offset may be determined by a number of seismic streamer cablestowed by the first seismic vessel and/or the second seismic vessel, therelative distance between the streamer cables, and the like.

In one embodiment, varying the distance between the first seismic vessel110 and the second seismic vessel 120 may involve varying the distance Oillustrated in FIG. 4B, thereby facilitating seismic data acquisition ata variety of azimuths. Any type of distance function, e.g., the distancefunctions illustrated in FIGS. 2A-C may be used to vary the offset O.While varying the distance along the offset O direction and the in-linedirection are disclosed herein, in alternative embodiments, the distancebetween the seismic vessels may be varied in any direction, e.g., thediagonal distance between the seismic vessels in FIG. 4B. In someembodiments, the distance between the seismic vessels may be variedalong two or more directions. For example, the distance between theseismic vessels may be varied along an in-line direction as well as anoffset direction.

FIG. 4C illustrates yet another method for conducting a seismic surveyaccording to an embodiment of the invention. As illustrated, a firstseismic vessel 110 may travel along a substantially straight navigationline L4. A second seismic vessel 120 may travel with the first seismicvessel 110 along a meandering path L5, as shown in FIG. 4C. Thevelocities of the vessels 110 and 120 may be selected and/or adjustedsuch that the distance between the vessels 110 and 120 is variedaccording to a predefined distance function. Furthermore, because thesecond seismic vessel 120 travels on a meandering path, seismic data iscollected along a variety of azimuths. For example, when the secondseismic vessel 120 is at a first position P1 seismic data may becollected at a first azimuth angle A1, whereas at a second position P2,seismic data may be collected at a second azimuth angle A2. While FIG.4C illustrates a first seismic vessel 110 travelling along a straightnavigation line and a second seismic vessel 120 travelling along ameandering path, in alternative embodiments, both seismic vessels 110and 120 may follow meandering paths.

Implementing the distance function D may be accomplished in a pluralityof ways. For example, in one embodiment, one of the seismic vessels 110and 120 may maintain a constant velocity while the other one of theseismic vessels 110 and 120 may vary its respective velocity to achievethe distance function. Alternatively, both of the seismic vessels maycoordinate adjustment of their respective velocities to achieve thedistance function. In embodiments where two or more seismic vessels varytheir respective velocities, each seismic vessel may vary its velocitywhile maintaining a predefined average velocity, according to oneembodiment. In one embodiment, the average velocity of each of thevessels may be substantially equal, but in other embodiments, differentaverage velocities may be defined for each vessel.

While embodiments of the invention are described with reference to twoseismic vessels 110 and 120, in alternative embodiments, any number ofseismic vessels may be utilized while conducting a survey. Furthermore,when more than two seismic vessels are utilized, distance functions maybe defined for any one or more pairs of seismic vessels.

In one embodiment of the invention, a plurality of seismic vessels mayrepeat acquisition over a particular sail line two or more times. Duringeach repetition, the distance function may be shifted such thatacquisition occurs at a variety of different receiver positions. FIG. 5Aillustrates three different shifted distance functions that may beimplemented over a particular sail/navigation line, according to anembodiment of the invention. A first distance function 510 may beimplemented during a first pass over a particular sail line, a seconddistance function 520 may be implemented during a second pass over thesame sail line, and a third distance function 530 may be implementedduring a third pass over the same sail line.

FIG. 5B illustrates exemplary positions of seismic receiver arrays in anarea of interest 550 during a two vessel operation according to anembodiment of the invention. For example, the area 561 may represent theposition of a first seismic receiver array associated with a firstseismic vessel during a first pass over a given sail line whileimplementing a first distance function, e.g., the distance function 510of FIG. 5A. The area 562 may represent the position of a second seismicreceiver array associated with a second seismic vessel during the firstpass over the sail line while implementing the first distance function.As shown in FIG. 5B, the distance between the first and second seismicreceiver arrays during the first pass may be dx.

During a second pass over the same sail line, a different distancefunction, e.g., the distance function 520 of FIG. 5A may be implemented.As a result, the distance between the first seismic receiver array andthe second seismic receiver array may be dy while in the area 550,therefore positioning the respective arrays in areas 571 and 572. Duringa third pass over the same sail line the seismic receiver arrays mayimplement a different distance function, e.g. function 530 of FIG. 5A,which may position the arrays in areas 581 and 582. If seismic data isrecorded when the receiver arrays are in the areas 561, 562, 571, 572,581, and 582 during the three passes, then seismic data may be gatheredfor the entire area 550.

Therefore, embodiments of the invention obviate the necessity ofexcessively large and long seismic arrays which can be difficult tomaneuver and manage by providing a method for varying relative positionsbetween multiple seismic arrays during consecutive passes over a sailline, thereby providing a requisite coverage of receiver positions forseismic data acquisition over a given area of interest.

FIG. 6 illustrates an exemplary control system 600 according to anembodiment of the invention. The control system may be configured toimplement a distance function between two seismic vessels as describedhereinabove. As illustrated in FIG. 6, the control system 600 mayinclude one or more processors 611, a memory 612, a global positioningsatellite (GPS) device 613, input/output devices 614, storage 615, and acommunications interface 616.

The input/output devices 614 may include input devices such as a mouse,keyboard, touchscreens, and the like, and output devices such as CRTmonitors, LCD displays, tablet computers, and the like. Storage device615 stores application programs and data for use by the control system600. Typical storage devices include hard-disk drives, flash memorydevices, optical media, network and virtual storage devices, and thelike. The communications interface 616 may connect the control system600 to any kind of data communications network, including either wirednetworks, wireless networks, or a combination thereof.

The memory 612 is preferably a random access memory sufficiently largeto hold the necessary programming and data structures of the invention.While memory 612 is shown as a single entity, it should be understoodthat memory 612 may in fact comprise a plurality of modules, and thatmemory 612 may exist at multiple levels, from high speed registers andcaches to lower speed but larger DRAM chips.

Illustratively, the memory 612 contains an operating system 617. Wellknown examples of operating systems include the Windows® operatingsystem, distributions of the Linux® operating system, and IBM's AIX andOS/2® operating systems, among others. More generally, any operatingsystem supporting the functions disclosed herein may be used.

Memory 612 is also shown containing a navigation program 618 which, whenexecuted by the processor 611, provides support for implementing adistance function during a seismic survey. For example, in oneembodiment, the navigation software may determine a position of one ormore seismic vessels conducting a survey (via e.g., GPS data from theGPS device 613) and based on the determined position, adjust a speed ofone or more of the seismic vessels to achieve a desired distancefunction.

Memory 612 may also contain distance functions 619 and survey data 620.The distance functions may define one or more distinct distancefunctions that may be used during a seismic survey, for example, thedistance functions illustrated in FIG. 3. Survey data 620 may includedata regarding, for example, specific areas of interest. In oneembodiment, the navigation software may use GPS data from the GPS device613 to determine whether one or more survey vessels are approaching anarea of interest (e.g., the area 310 shown in FIG. 3), and in responseto determining that the one or more vessels are approaching an area ofinterest, adjust the distance function that is used, as describedhereinabove.

In one embodiment of the invention, the control system 600 may beimplemented in computer systems included in one or more seismic vesselsconducting the survey. For example, navigation software systemsoperating in each vessel may communicate with each other via thecommunications interface 616 to adjust the distance between one or morevessels, thereby achieving a desired distance function. The navigationsoftware operating in multiple vessels may use any reasonable model ofcommunication, for example, a master-slave configuration whereinnavigation software in one primary vessel controls the navigationsoftware in other vessels.

In another embodiment, the control system 600 may not be located on theseismic vessels. For example, the seismic vessels may be configured tocommunicate with a central computer system which may be located, forexample, on land, and receive instructions from the central computersystem to adjust speeds to achieve a desired distance function.

FIG. 7 is a flow diagram of exemplary operations that may be performedduring a seismic survey, according to an embodiment of the invention.The operations generally comprise operating a first seismic vesselconfigured to tow at least one of one or more first seismic sources andone or more first streamers comprising a plurality of seismic receivers,as illustrated in step 710. The operations further comprise operating asecond seismic vessel configured to tow at least one of one or moresecond seismic sources and one or more second streamers comprising aplurality of seismic receivers, as illustrated in step 720. In step 730,a distance between the first seismic vessel and the second seismicvessel may be adjusted according to a predefined distance functionduring seismic data acquisition.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A method for acquiring seismic data, comprising: operating a firstseismic vessel towing at least one of a first seismic streamercomprising a first plurality of seismic sensors and a first source; andoperating a second seismic vessel towing at least one of a secondseismic streamer comprising a second plurality of seismic sensors and asecond source; wherein the first seismic vessel follows a first seismicacquisition line and the second seismic vessel follows a second seismicacquisition line that is offset from the first seismic acquisition line;and during acquisition of the seismic data, adjusting an in-linedistance between the first seismic vessel and the second seismic vesselaccording to a first predefined function.
 2. The method of claim 1,wherein the first seismic vessel and the second seismic vessel travelalong substantially straight seismic acquisition lines.
 3. (canceled) 4.The method claim 1, further comprising: repeating the acquisition of theseismic data along a previously traveled path by the first seismicvessel and the second seismic vessel; and during the repeatedacquisition, adjusting the in-line distance between the first seismicvessel and the second seismic vessel according to a second predefinedfunction.
 5. (canceled)
 6. The method of claim 1, wherein adjusting thein-line distance between the first seismic vessel and the second seismicvessel comprises adjusting a speed of one or more of the seismic vesselsto achieve a desired distance function by cyclically: reducing thein-line distance between the first seismic vessel and the second seismicvessel; and increasing the in-line distance between the first seismicvessel and the second seismic vessel.
 7. The method of claim 1, whereinadjusting the in-line distance between the first seismic vessel and thesecond seismic vessel comprises: operating the first seismic vessel at aconstant first velocity following the first seismic acquisition line;and operating the second seismic vessel at a variable second velocityfollowing the second seismic acquisition line.
 8. (canceled)
 9. Themethod of claim 1, wherein: the first seismic vessel tows the firstsource and the second seismic vessel tows the second source; and thefirst and second sources are configured to fire substantiallysimultaneously.
 10. A non-transitory computer readable storage mediumcarrying a program product comprising computer-readable instructionswhich, when executed on a computer processor, are configured to performa method comprising: operating a first seismic vessel towing at leastone of a first seismic streamer comprising a first plurality of seismicsensors and a first source; and operating a second seismic vessel towingat least one of a second seismic streamer comprising a second pluralityof seismic sensors and a second source; wherein the first seismic vesselfollows a first seismic acquisition line and the second seismic vesselfollows a second seismic acquisition line offset from the first seismicacquisition line; and during acquisition of seismic data, adjusting anin-line distance between the first seismic vessel and the second seismicvessel according to a first predefined function.
 11. The computerreadable storage medium of claim 10, wherein the first seismic vesseland the second seismic vessel travel along substantially straightseismic acquisition lines.
 12. (canceled)
 13. The computer readablestorage medium claim 10, the method further comprising: repeating theacquisition of the seismic data along a previously traveled path by thefirst seismic vessel and the second seismic vessel; and during therepeated acquisition, adjusting the in-line distance between the firstseismic vessel and the second seismic vessel according to a secondpredefined function.
 14. (canceled)
 15. The computer readable storagemedium of claim 10, wherein adjusting the in-line distance between thefirst seismic vessel and the second seismic vessel comprises adjusting aspeed of one or more of the seismic vessels to achieve a desireddistance function by cyclically: reducing the in-line distance betweenthe first seismic vessel and the second seismic vessel; and increasingthe in-line distance between the first seismic vessel and the secondseismic vessel.
 16. The computer readable storage medium of claim 10,wherein adjusting the in-line distance between the first seismic vesseland the second seismic vessel comprises: operating the first seismicvessel at a constant first velocity following the first acquisitionline; and operating the second seismic vessel at a variable secondvelocity following the second seismic acquisition line.
 17. (canceled)18. The computer readable storage medium of claim 10, wherein: the firstseismic vessel tows the first source and the second seismic vessel towsthe second source; and the first and second sources are configured tofire substantially simultaneously.
 19. A seismic vessel configured to:tow at least one of one or more seismic streamers and a seismic source;and during acquisition of seismic data, adjust a speed of the seismicvessel to achieve a desired in-line distance from another seismic vesselaccording to a distance function; wherein the seismic vessel isconfigured to travel at a variable velocity following a first seismicacquisition line to adjust the in-line distance from the other seismicvessel following a second seismic acquisition line that is offset fromthe first seismic acquisition line; wherein the in-line distance isreduced from a first distance to a second distance during a first timeperiod and increased from the second distance to the first distance in asecond time period. 20-26. (canceled)
 27. The seismic vessel of claim19, further configured to: determine a position of the seismic vessel;and based on the determined position, adjust a speed the seismic vesselto achieve the distance function.
 28. The seismic vessel of claim 19,further configured to: determine whether the seismic vessel isapproaching an area of interest; and in response to determining that theseismic vessel is approaching an area of interest, adjust the distancefunction.
 29. The seismic vessel of claim 19, wherein the first andsecond seismic acquisition lines are parallel.
 30. The method of claim1, wherein the first and second seismic acquisition lines are parallel.31. The method of claim 1, wherein an offset between the first seismicacquisition path and the second seismic acquisition path is determinedby a number of seismic streamer cables towed by the first seismic vesselor the second seismic vessel.
 32. The computer readable storage mediumclaim 10, wherein the first and second seismic acquisition lines areparallel.
 33. The computer readable storage medium claim 10, wherein anoffset between the first seismic acquisition path and the second seismicacquisition path is determined by a number of seismic streamer cablestowed by the first seismic vessel or the second seismic vessel.